In a communications system where signals are transmitted over a communications channel, the transmitted signals are normally corrupted by imperfections in the channel. In order to properly recover the transmitted signals, circuits which correct various signal impairments are employed in a receiver. Phase jitter is one such signal impairment. It is typically modeled as a set of sinusoids of different frequencies and is attributed to power line harmonics and ringing voltages present in the channel.
Phase-jitter-correction techniques typically call for an estimation of the phase jitter before the jitter can be effectively removed. One such technique involves the use of an adaptive finite-impulse-response (FIR) filter for the phase jitter estimation, as described in U.S. Pat. No. 4,320,526 issued Mar. 16, 1982 to R. D. Gitlin, which is hereby incorporated by reference. Disadvantageously, such an FIR filter is relatively slow in converging on each of the phase jitter frequencies during an adaptation process. Moreover, this FIR filter does not provide the estimate accurate enough to effect a substantial removal of the phase jitter.
Another technique involves the use of an adaptive IIR filter for estimating the phase jitter, as described in U.S. Pat. No. 4,847,864 issued Jul. 11, 1989 to R. L. Cupo, which is hereby incorporated by reference. The coefficients of such an IIR filter are selected, using a well-known least-mean-square (LMS) optimization method, to minimize a phase error indicative of the difference between the phase jitter estimate and the actual phase jitter present in the received signal. Stemming from certain limitations of the optimization method used, this IIR filter can only provide an estimate, although accurate, of a particular frequency component of the phase jitter. However, in practice, the phase jitter oftentimes consists of a plurality of frequency components. This being so, in order to remove the phase jitter completely, a multiplicity of these IIR filters (i.e., one filter for each frequency component) must be incorporated in the receiver. Notwithstanding such, the actual number of jitter frequency components incurred in a channel varies from one channel to another and, therefore, cannot be determined in advance. Overestimating the number of the jitter frequency components results in incorporating too many such filters and adds an unnecessary, substantial cost to the system. On the other hand, underestimating the number results in too few such filters and undesirably degrades the system's performance.
Accordingly, it is desirable to have a phase-jitter-correction arrangement capable of removing all of the jitter frequency components from the received signal, regardless of their number.